The present invention generally relates to mixing fluids. More specifically, the invention relates to the continuous mixing of two fluids in a consistent proportion into an exit stream.
The proportional mixing of fluids by automatic, self-powered devices is known. Such a device can be used to mix a fluid stream, such as water, with another fluid such as drugs, vaccines, nutrients, treating agents or the like. Traditional self-powered proportioners have a fluid motor and slave pump.
In a traditional self-powered proportioner, the pressure of a first fluid stream, such as water, is used to power the device. The first fluid stream cyclically moves a motor piston within a cylinder, automatically metering an amount of the first fluid with each stroke by filling the cylinder with fluid. A gating device alternates the fluid intake from one side of the motor piston to the other. Intake fluid pressure causes the motor piston to move, while fluid on the opposite side of the motor piston is simultaneously expelled. In one cycle of the proportioner, two displacements of the first fluid are expelled from the motor cylinder into the exit stream.
In known proportioners, the motor piston powers a slave pump. The slave pump has a slave piston and slave cylinder. The slave piston has one working side, compared to the motor piston which has two. The slave pump draws in an amount of a second fluid through an intake, then expels it through an outlet into the exit stream. The expelled second fluid mixes with the first fluid expelled from the motor piston. In one cycle, one displacement of fluid from the slave cylinder is expelled into the exit stream. Thus, constant proportions of the two fluids are mixed in each cycle. Such a proportioner is described in U.S. Pat. No. 4,572,229, issued to Thomas D. Mueller, which is incorporated herein by reference. The present invention is an improvement thereof.
In known proportioners, the slave piston is driven by a connecting rod that is connected to the motor piston. In the proportioner disclosed in U.S. Pat. No. 4,572,229, a cavity in the housing surrounding the connecting rod acts as a fluid conduit between the motor cylinder and the back of the slave piston. Thus, the back of the slave piston is exposed to the same fluid pressure as one side of the motor piston.
The fluid pressure on the back side of the slave piston imposes a force opposite to that of the motor piston. However, so long as the motor piston has a significantly larger area than the slave piston, the motion of the proportioner is not significantly impeded, despite the inefficiency caused by the back-pressure.
Proportioners have typically been used to mix fluids in a fairly high ratio, e,g., 50 parts water to 1 part additive. The slave piston is used for adding the lesser quantity of fluid. For such a ratio, the area of the slave piston is relatively small compared to that of the motor piston. A small slave piston area creates a relatively small back-pressure which can easily be overcome by the larger motor piston. Therefore, with such sizing, such a proportioner can work effectively.
However, for certain applications, there is a need to mix fluids in ratios of 4:1 or lower. To achieve lower mixing ratios, the slave piston must have a larger area than in the proportioner described above. A larger slave piston area results in a larger inefficiency, because a larger slave piston has a larger back-pressure force acting on it. If the slave piston is too large in relation to the motor piston, the power of the fluid motor cannot overcome the slave piston back-pressure and internal friction, stalling the proportioner. Thus, it has been impractical to use such a proportioner for mixing two fluids in low ratios.
A need, therefore, exists for a proportioner with maximized efficiency. A need also exists for a self-powered proportioner that can mix fluids in low ratios.